The availability of low cost natural gas has led to the restart and construction of numerous ammonia production facilities throughout North America. Ammonia is typically produced through steam methane reforming. In the steam methane reforming process, air is used to auto-fire the reaction and to supply nitrogen for the ammonia synthesis reaction. In general, the steam methane reforming based process consists of primary steam reforming, secondary ‘auto-thermal’ steam reforming followed by a water-gas shift reaction and carbon oxide removal processes to produce a synthesis gas. The synthesis gas is subsequently dried to produce a raw nitrogen-hydrogen process gas with small amounts of methane and inerts which is then fed to an ammonia synthesis reaction. In many ammonia production plants, the raw nitrogen-hydrogen process gas is often subjected to a number of purification or additional process steps prior to the ammonia synthesis reaction.
A commercially important part of the ammonia processing train often used in ammonia plants is a cryogenic purification process known by those skilled in the art as the ‘Braun Purifier’. Since the secondary reformer is fed with an air flow having a nitrogen content that is larger than that required by the stoichiometry of the ammonia synthesis reaction, excess nitrogen, unconverted methane and inert gases must be removed or rejected from the raw nitrogen-hydrogen process gas prior to the ammonia synthesis step. In order to reject the excess nitrogen, unconverted methane and inerts, the Braun-type cryogenic purification process is introduced after the methanation reaction. The primary purpose of this Braun-type cryogenic purification process is to generate an overhead ammonia synthesis gas stream with a stoichiometric ratio of hydrogen to nitrogen of about 3:1 and low levels of methane and inerts.
The cryogenic purification step of the Braun Purifier typically employs a single stage of refrigerated rectification. The overhead synthesis gas stream from the single stage of refrigerated rectification is substantially free of unconverted methane and a substantial portion of the inerts, such as argon, are rejected into the fuel gas stream-bottoms liquid. In the Braun Purifier process, the feed gas stream is first cooled and dehydrated. The feed gas stream is then partially cooled and expanded to a lower pressure. The feed gas stream may be further cooled to near saturation and partially condensed and then directed to the base of the single stage rectifier. The rectifier overhead is the resulting ammonia synthesis gas that is processed for ammonia synthesis, whereas the rectifier bottoms are partially vaporized by passage through the rectifier condenser and warmed to ambient temperatures. This fuel/waste stream is typically directed back to the reformer and serves as fuel. See Bhakta, M., Grotz, B., Gosnell, J., Madhavan, S., “Techniques for Increase Capacity and Efficiency of Ammonia Plants”, Ammonia Technical Manual 1998, which provides additional details of this Braun Purifier process.
The waste gas from the Braun Purifier process step is predominantly a mixture of hydrogen (6.3 mole %), nitrogen (76.3 mole %), methane (15.1 mole %) and argon (2.3 mole %). The conventional argon recovery processes from ammonia tail gas are typically integrated with the hydrogen recovery process downstream of the Braun purifier. The conventional argon recovery processes are relatively complex and involves multiple columns, vaporizers, compressors, and heat exchangers, as described for example in W. H Isalski, “Separation of Gases” (1989) pages 84-88. Other relatively complex argon recovery systems and process are disclosed in U.S. Pat. Nos. 3,442,613; 5,775,128; 6,620,399; 7,090,816; and 8,307,671. Similarly, systems and processes for the recovery of argon, hydrogen and nitrogen from the waste gas are disclosed in U.S. Pat. Nos. 3,666,415; 3,675,434; 4,058,589; 4,077,780; 4,524,056; 4,752,311 and United. States Patent Application Publication No. 2013/0039835; and 2016/0060130. While these waste gas processing solutions adequately recover the argon, hydrogen and nitrogen, they do so at additional capital and operating costs.
What is needed therefore is an efficient and cost effective solution for recovery of the hydrogen, methane, nitrogen, and argon that is preferably integrated with the cryogenic purification of the synthesis gas.